1. Field of the Invention
The present invention relates to a projection optical system, for example, provided in a projection exposure apparatus that is used on occasion of photolithography when manufacturing semiconductor elements, display elements such as liquid crystal display and plasma display, and micromachine etc. such as microdevices. It also relates to a manufacturing method of the projection optical system and to a projection exposure apparatus,
2. Related Background Art
In a photolithographic process used for manufacturing semiconductor elements and the like, projection exposure apparatus is used to expose, through a projection optical system, the image of the pattern of the reticle as mask onto a substrate, i.e. a wafer (or glass plate, etch) on which is applied a photo resist. As the degree of integration of semiconductor elements and so on continue to improve, demand for a resolving power of the projection optical system in a projection exposure apparatus becomes even greater. In order to meet the demand, shortening the wavelength of the illuminating light (exposure light) for exposure and increasing the numerical aperture (NA) of the projection optical system have been performed. However, when exposure a light wavelengths are shortened, varieties of glass material can be used practically are becoming fewer, due to the light absorption. In particular, when the exposure light wavelength becomes vacuum ultraviolet VUV), approximately 200 nm or shorter, the glass materials that can be used under current conditions becomes limited to, for example, synthetic silica, fluorite (CaF2), and magnesium fluoride (MgF2). The problem of how to compensate chromatic aberration therefore happens.
Even so, there has been progress made in the practical application of the refractive system in projection optical systems using an ArF excimer laser (wavelength 193 nm) as exposure light. However, when the exposure light wavelength fall to 180 nm or shorter as with an F2 laser (wavelength 157 nm), it is difficult to put projection optical systems into practical use with a refractive system, but expectations are rising for catadioptric systems providing a reflective member as means for compensating chromatic aberration. In this regard, since it is known that fluorite has sufficient transmissivity up to approximately 100 nm, it may be used as a refractive member up to this range. A catadioptric optical system is realized by assembling a refractive member made of fluorite (calcium fluoride) and a reflective member.
A number of types of catadioptric optical systems have already been proposed. Of these, optical systems that have the center portion of the aperture stop shielded are considered most influential since, by using a reflective surface having two or more faces, all of the optical elements can be disposed in an upright cylindrical shape along a single the optical axis without requiring an optical path deflection member. Moreover, since the image of an object in the optical axis can be formed on an image plane, they also include the advantage of chromatic aberration being able to be compensated in a wide exposure field with only a relatively low number of optical elements. Of these, optical systems that form an intermediate image partway through the optical system, for example, such as the optical system disclosed in U.S. Pat. No. 5,650,877, are quite effective in terms of throughput and ease in manufacturing in comparison with optical systems that have an optical member such as a semi-transparent mirror disposed partway, and therefore have no intermediate image formed, greatly reduced light intensity, and increased danger of large-scale flares developing. This type or conventional technology is cited in, for example, U.S. Pat. Nos. 5,717,518 and 5,488,229.
In addition, slice the optical absorption rate of the exposure light increases due to oxygen and carbon dioxide when exposure light is approximately 200 nm or shorter, in order to increase the illuminance on the wafer, it is necessary to replace the air within the lens barrel of the projection optical system with a purge gas such as nitrogen gas (N2) or helium gas (He), which have high transmissivity for wavelengths of approximately 200 nm or shorter. To this end, various purge gas supply mechanisms have conventionally been proposed.
The present invention was devised as a result of the following findings by the inventors. As shown with the above, in cases where an exposure light belonging to vacuum ultraviolet region is used, a catadioptric optical system is an excellent projection optical system. However, with the catadioptric optical system, even if a virtually upright cylinder, the entire length is quite long in comparison with a refractive system, and becomes difficult to support with a single lens barrel. Furthermore, with the catadioptric optical system in U.S. Pat. No. 5,717,518, as it can be seen from the fact that the material for the reflective members is large compared to the glass used for the refractive members, with the catadioptric optical systems it is often the case that the reflective members (material) rust be made larger than the refractive members (glass). In such cases, if a single lens-barrel with a relatively small refractive member and a relatively large reflective member are supported as a single body, the configuration of that lens barrel becomes complicated, and the stable support of that lens barrel becomes difficult. Moreover, there are problems with production costs becoming high with projection optical systems since it takes time to perform the assembly adjustment of that lens barrel and all of the optical elements.
The method for holding the optical element group that comprises catadioptric optical system not by a single lens-barrel, but divided between a plurality of divided lens-barrels is also considered. However, in cases where these optical elements are simply divided into a plurality of groups and held by a plurality of divided lens-barrels, there require the divided lens-barrels with partially extremely complex structures, and also, tears are entertained as to complex processes of assembly adjustment such as aligning the respective axes of each divided lens-barrel to each other.
Furthermore, since there is a danger of the positional relationship between optical elements that are held by different divided lens-barrels fluctuating relative to set values by simply holding a plurality of divided lens-barrels, there is a danger of an increase in contributing factors for the development of aberrations. In cases where a plurality of divided lens-barrels are used, there require positioning, for example, that suppresses the development of aberrations
Moreover, in the projection exposure apparatus using vacuum ultraviolet light having wavelengths of approximately 200 nm or less as exposure light, when a purge gas is flowed through the optical path inside of projection optical systems, if those lens barrels are configured from a plurality of divided lens-barrels, then stagnancy develops in the flow of purge gas at the boundaries of the divided lens-barrels, and there is a danger that the density of light absorbing substances, such as oxygen, may not sufficiently decrease. In particular, in a projection exposure apparatus using light of a wavelength of 170 nm or shorter, for example F2 laser light (157 nm wavelength) as exposure light, since the permissible value of the residual density of the light absorbing substances is lowered, if a divided lens-barrel type is simply adopted, there is a danger that the residual density of the light absorbing substances may not fall below the permissible value,
An object of the present invention is to provide a projection optical system capable of obtaining high optical performance, a manufacturing method thereof, and a projection exposure apparatus in which it is included.
Another object of the present invention is to provide, in addition to the above objects, a projection optical system that includes a catadioptric optical system having simple assembly adjustment, a manufacturing method thereof, and a projection exposure apparatus in which it is included.
Yet another object of the present invention is to provide, in addition to the above objects, a projection optical system that includes a catadioptric: optical system that is capable of adjusting the state of the optical elements, a manufacturing method thereof, and a projection exposure apparatus in which it is included.
Yet another object of the present invention is to provide, in addition to the above objects, a projection optical system that includes a catadioptric optical system capable of maintaining a stable positional relationship between a plurality of optical elements, a manufacturing method thereof, and a projection exposure apparatus in which it is included.
Yet another object of the present invention is to provide, in addition to the above objects, a projection optical system capable of supplying a high-purity purge gas and a projection exposure apparatus in which it is included.
An aspect of the present invention is a projection optical system forming an image of an object in a first plane onto a second plane comprising, an optical element group including at least one refractive member and a plurality of reflective members, and a plurality of lens-barrel units holding the optical element group divided into a plurality of groupings, wherein the plurality of reflective members is all held by one lens-barrel unit of the plurality of lens-barrels units.
Lens-barrel units hold an imaging optical system for forming an image (for example, an intermediate image or final image), and are configured from, for example, a plurality of divided lens-barrels. Each of the lens-barrel units has a mechanism to adjust the state of the imaging optical system. In the following, divided lens-barrels have the same meaning as lens-barrel units.
An aspect of the present invention is that it aims at making the amount of aberrations that are generated in cases where the position of the optical elements shifts from a designed position, higher in the reflective members than in the refractive members. According to an aspect of the present invention, since all of a plurality of reflective members is held in one lens-barrel unit, the relative positional relationship of the plurality of reflective members can be maintained in a nearly stable state. Also, even if the lens-barrel unit holding the reflective members is displaced due to vibrations, for example, if the lens-barrel unit is thought of as a reference, since it is the same value as the only displaced lens-barrel unit holding only another refractive member, the entire amount of generated aberration is suppressed and high optical performance can be obtained.
In this case, it is preferable that the partial optical element group forming an intermediate image on a third plane between the first plane and the second plane, wherein the partial optical element group is integrally held by one lens-barrel unit of the plurality of lens-barrel units. When an intermediate image is formed as such, the outer diameter of the optical elements bath before and after the intermediate image may vary greatly. At this time, by having the partial optical group that forms the intermediate image be held by one lens-barrel unit, the partial optical element group can be easily and stably held.
Furthermore, as an example, that optical element group is disposed along a single the optical axis. Through such configuration in a vertical barrel, the entire configuration of the lens barrel becomes smaller, and that optical element group can be stably held. It is noted here that, as an another example, that optical element group may include a first partial optical element group disposed along a first optical axis, a second partial optical element group disposed along a second optical axis which extends in a direction that intersects the first optical axis, and is optically connected to the first partial optical element group, and a third partial optical element group disposed along a third optical axis which extends in the same direction as the first optical axis, and is optically connected to the second partial optical element group. Here a second optical axis, which is approximately orthogonal to the first optical axis, is included in a second optical axis that extends along the direction that intersects a first optical axis. A third optical axis that is approximately parallel to first optical axis is included in a third optical axis that extends in the same direction as first optical axis.
Furthermore, it is preferable that the plurality of lens-barrel units be supported independent of each other. As a result of this, the assembly adjustment is simplified.
Furthermore, it is preferable that at least one of said plurality of lens-barrel units, has an adjustment mechanism to adjust a predetermined state of optical elements respectively held therein. By adjusting the predetermined state (positioning of the optical axis direction, the position of the direction of the two axes within the plane that is orthogonal to the optical axis (decentering adjustment), and the tilt angle around the two axes) of those optical elements, the magnification and predetermined imaging performance, such as aberrations, of that projection optical system can be compensated.
Furthermore, it is preferable that at least one of the plurality of lens-barrel units further comprising a plurality of holding blocks which hold one or a plurality of optical elements, wherein the plurality of holding blocks have an adjustment mechanism to adjust the state of optical elements respectively held therein.
Furthermore, it is preferable that at least one of the plurality of lens-barrel units be detachable.
An aspect of the present invention, as an example, uses an illumination light with wavelengths of 200 nm or shorter, preferably supplies a purge gas through which the illumination light passes to inside the projection optical system. Since each projection optical system of the present invention comprises a plurality of lens-barrel units or a plurality of holding blocks that are built up (connected), assembly adjustment is simplified. Moreover, since these lens-barrel units and holding block can be configured with a high level of gas-tightness, a high-purity purge gas can be supplied into those projection optical systems. The illuminance of the illumination light becomes high and a high throughput can be accordingly obtained for the exposure process.
It is preferable that there be provided a plurality of vents (openings) in the part (lens mount, etc.) that holds each optical element (refractive member, reflective member) within each lens-barrel unit or each holding block as a porous structure. Furthermore, a holding block that holds, for example, a relatively large reflective member, may be supported by a frame mechanism having high ventilation. By having these mechanisms there is no settlement from the purge gas, and as a result it is possible to reduce the residual density of impurities (light absorbing substances) within the projection optical system.
Another aspect of the present invention is a projection optical system comprising, an optical element group including first and second optical element sub-groups, and forming an image of a predetermined magnification of an object in a first plane onto a second plane, a first lens-barrel unit integrally holding the first optical element sub-group as a single body along a first optical axis, and a second lens-barrel unit integrally holding the second optical element sub-group as a single body along a second optical axis that is coaxis with the first optical axis, wherein the first lens-barrel unit is held in a position at a plane that is orthogonal to the first optical axis and that passes through a point divides a line, along said first optical axis, between an object point and an image point relative to the first sub optical element group by 1:xcex21 (where xcex21 is a real number other than zero) or in a position near the plane, and the second lens-barrel unit is held in a position at a plane that is orthogonal to the second optical axis and that passes through a point divides a line, along said second optical axis, between an object point and an image point relative to the second sub optical element group by 1:xcex22 (where xcex22 is a real number other than zero), or in a position near said plane.
According to another aspect of the present invention, because each lens-barrel unit is independently supported relative to each other, assembly adjustment is simplified. Moreover, even if first lens-barrel unit and second lens-barrel first are tilted, the respective image shift hardly occurs. When used in a projection exposure apparatus even if lens-barrel unit is displaced because of the affect of vibrations of the stage system and so on, high optical performance can always be obtained. This aspect can also be used as a limiting factor of an aspect of the present invention above,
Yet another aspect of the present invention is a projection optical system which forms an image of an object in a first plane onto a second plane, comprising, an optical element group having a plurality of aspherical surfaces, and a plurality of holding blocks holding the optical element group in a plurality of groupings, wherein the number of the plurality of holding blocks is equal to or greater than the number of the aspherical surfaces.
When assembling the projection optical system according to another aspect of the present invention, there may be a case where several xcexcm of decentering remains in each optical element where an aspherical surface is formed. Due to such decentering of the aspherical surface, a high order decentered aberration occurs. For example by performing decentering adjustment when successively integrating, or by providing an decentering adjustment mechanism in each holding block, the number of holding blocks is kept equal to or greater than the number of aspherical surfaces, it is possible to compensate that high-order decentered aberration and obtain high optical performance.
In this case, when those optical element groups have a plurality of reflective members, it is preferable that this plurality of reflective members be held in holding blocks that are different from each other. When the position of the reflecting members is off a designed position, a large aberration compared to the reflective member occurs. When a plurality of reflective members are provided, it is possible to minimize aberration by holding each reflecting member in a different holding block and, for example, adjusting the relative position when integrating these holding blocks, or having a mechanism that can fine tune the relative position of the reflecting lens. This aspect can also be used as a restriction of an aspect of the present invention above.
Yet another aspect of the present invention is a projection optical system which forms an image of an object in a first plane onto a second plane comprising, an optical element group, a plurality of lens-barrel units, which hold said optical element group in a plurality of groupings, and frame from which at least one lens-barrel unit of said plurality of lens-barrel units is held so as to hang down.
By supporting the lens-barrel unit so as to hang down in this manner, it is possible to provide constant, stable support to the lens-barrel unit and the optical elements that are held therein, and maintain high optical performance. This aspect can also be used as a restriction of an aspect of the present invention above.
Yet another aspect of the present invention is a projection optical system forming an image of an object in a first plane onto a second plane by using light with wavelengths of 200 nm or shorter, comprising, at least two refractive members disposed in the optical path of the light, at least two lens-barrel units aligning the at least two refractive members, a pipeline being connected to at least one lens-barrel unit of the at least two lens-barrel units and supplying purge gas through which the light can pass within the space between the at least two refractive members, and at least two holding units being included in each of the at least two lens-barrel units and holding each of the at least two refractive members, wherein the at least two holding units having openings to allow the purge gas to pass through.
By having these mechanisms, there is no settlement from the purge gas, and as a result it is possible to reduce the residual density of impurities (light absorbing substances) in the projection optical system. Not only is this aspect a catadioptric projection optical system, but it can also be suited for a dioptric optical system. In this aspect, it is preferable that it has light of a wavelength of equal to 160 nm or shorter. This aspect can also be used as a restriction of an aspect of the present invention.
Yet another aspect of the present invention is a projection optical system provided in a projection exposure apparatus, comprising, an imaging optical system including all of a plurality of reflective members which are structural components of the projection optical system, another imaging optical system including at least one refractive member which is a structural component of the projection optical system and not including a reflective member which is a structural component of the projection optical system, a lens-barrel unit holding the imaging optical system, and another lens-barrel unit holding the another imaging optical system.
The sane things can be said as the aspects of the present invention described above accordingly. A lens-barrel unit comprises a plurality of divided lens-barrels, which include a mechanism for adjusting the state of imaging optical systems, and the other lens-barrel unit can be comprised of a plurality of divided lens-barrels, which includes a mechanism for adjusting the state of the other imaging optical system. Furthermore, each of the plurality of reflective members can be held by a divided lens-barrel that is different from the others of a plurality of divided lens-barrels, which forms a lens-barrel unit. This aspect can also be used as a restriction of an aspect of the present invention above.
Another aspect of the present invention is a method of manufacturing a projection optical system which comprises an optical element group having at least one refractive member and a plurality of reflective members, and a plurality of lens-barrel units holding the optical element group as a plurality of groupings, the method of manufacturing a projection optical system comprising, a first step of pre-setting a predetermined lens-barrel unit of the plurality of lens-barrel units to be detachable, putting together a lens-barrel unit to be adjusted at the position of the predetermined lens-barrel unit of a first projection optical system that is already completed as the projection optical system, and performing adjustment on the lens-barrel unit to be adjusted while measuring optical characteristics of the first projection optical system, and a second step of building up a second projection optical system by an adjusted lens-barrel unit on which adjustment is performed in the first step and lens-barrel units other than this, and performing adjustment of the second projection optical system using the adjusted lens-barrel unit as a standard.
With the manufacturing method of a projection optical system according to yet another aspect of the present invention, when manufacturing a plurality of projection optical systems in parallel, for example, it is possible to adjust lens barrel unit of the next projection optical system in only a short time by using projection optical system, which has completed assembly adjustment. Thereby, the assembly adjustment time can be shortened as a whole.
Yet another aspect of the present invention is a method of manufacturing a projection optical system which comprises an optical element group including at least one refractive member and at least one reflective member, a plurality of lens-barrel units holding the optical element group as a plurality of groupings, a holding block being a component of at least one lens-barrel unit of the plurality of lens-barrel units and holding at least one optical element, wherein the holding block is adjustable in at least one of relative orientation and relative position to the lens-barrel unit including the holding block as structural component, and at least one entire lens-barrel unit of the plurality of lens-barrel units is adjustable, the method comprising the step of; a first step of adjusting the plurality of lens-barrel units relative to each other; and a second step of adjusting the at least one of the relative orientation and the relative position of the holding block, so as to adjust residual aberrations, after the first step.
By performing adjustment in this manner, fine-tuning can be performed on large and small aberrations, and it is possible to manufacture the projection optical system that has optical performance nearly equal that of the designs.
Yet another aspect of the present invention is a projection exposure apparatus, which comprises the projection optical system of the present invention mentioned above and which projects an image of a projection original disposed on the first plane onto a workpiece, with the projection optical system. By using the projection optical system of the present invention, the assembly adjustment of the entire apparatus becomes easy and the manufacturing cost can be reduced.
Yet another aspect of the present invention is a projection exposure method, which projects an image of a projection original disposed on the first plane onto a workpiece, with the projection optical system of the present invention mentioned above. By using the projection optical system of the present invention, the assembly adjustment of the entire apparatus becomes easy and the manufacturing cost can be reduced.
The present invention will be more fully understood from the detailed description given herein below and the accompanying drawings, which are given by way of illustration only and are not to be considered as restricting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.